docs/parse-support-md: Correctly process caveats in multi-status sections

docs/parse-support-md: Correctly process caveats in multi-status sections

When SUPPORT.md uses the syntax
  Status, <some thing>: <support status>
the caveats were lost (not footnoted) because they were attached
only to <some thing>.

Caveats occur in running text, so they are necessarily part of a real
section, not an individual status line like that.  So attach them to
the RealSectNode, and look there for them.

Reported-by: Lars Kurth <lars.kurth@citrix.com>
Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
Acked-by: Lars Kurth <Lars.kurth@citrix.com>
Release-acked-by: Juergen Gross <jgross@suse.com>